Semiconductor memory integrated circuit and its manufacturing method

ABSTRACT

A method of manufacturing a semiconductor memory integrated circuit intended to improve properties and reliability of its peripheral circuit includes the step of forming a tunnel oxide film ( 21   a ) in the cell array region, gate oxide film ( 21   b ) for a high-voltage circuit and gate oxide film ( 21   c ) for a low-voltage circuit both in the peripheral circuit to respectively optimum values of thickness, and covering them with a first-layer polycrystalline silicon film ( 22 ). After that, device isolation grooves ( 13 ) are formed and buried with a device isolation insulating film ( 14 ). The first-layer polycrystalline silicon film ( 24 ) is a non-doped film, and after device isolation, a second-layer polycrystalline silicon film ( 24 ) is doped with phosphorus in the cell array region to form floating gates made of the first-layer polycrystalline silicon film ( 22 ) and the second-layer polycrystalline silicon film ( 24 ). In the peripheral circuit, gate electrodes are made of a multi-layered film including the first-layer polycrystalline silicon, film ( 22 ), second-layer polycrystalline silicon film ( 24 ) and third-layer polycrystalline silicon film  28 , and impurities are ion implanted thereafter to respective transistor regions under respectively optimum conditions.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of and claims the benefit of priorityunder 35 USC §120 from U.S. application Ser. No. 10/938,514, filed Sep.13, 2004, now allowed, which is a division of U.S. Pat. No. 6,794,708,issued Sep. 21, 2004, and is based upon and claims the benefit ofpriority under 35 USC §119 from the prior Japanese Patent ApplicationNo. 2000-174127, filed Jun. 9, 2000 and Japanese Patent Application No.2001-171612, filed Jun. 6, 2001; the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor memory integrated circuit madeup of a cell array with an arrangement of electrically erasable andprogrammable nonvolatile memory cells and a transistor circuit disposedaround the cell array (peripheral circuit), and a manufacturing methodof the semiconductor memory integrated circuit.

2. Description of the Related Art

Memory cells of EEPROM flash memory has a transistor structure includingstacked floating gates and control gates. The floating gate of such amemory cell is commonly made of a polycrystalline silicon film dopedwith phosphorus to an adequate concentration. Phosphorus concentrationof the floating gate affects the quality of an underlying tunnelinsulating film and the configuration of the floating gate itself bypost thermal oxidation. Since the quality of the tunnel insulating filmand the floating gate configuration significantly influence the propertyand reliability of the memory cell, they need be properly controlledindependently from other parameters.

On the other hand, the transistor circuit disposed around the cell array(hereinbelow simply called peripheral circuit) is made by using a CMOSstructure at least in a logic circuit. To ensure that transistors of theperipheral circuit exert their performance required as surface channeltype transistors, it is necessary to dope a p-type impurity {typicallyboron) into gate electrodes in case of MOS transistors or an n-typeimpurity (typically arsenic) into gate electrodes in case of NMOStransistors. Additionally, to prevent depletion of gates, a doped amountnot less than a predetermined concentration and activation of theimpurity are indispensable.

Taking account of these requirements for such a cell array and itsperipheral circuit, for conventional flash memory, the followingmanufacturing process is used. FIGS. 35(a) through 35(d) show majorsteps noticing the cell array region. As shown in FIG. 35(a), a siliconsubstrate 1, having formed a tunnel oxide layer 2 thereon and apolycrystalline silicon film 3 a on the tunnel oxide layer 2, isseparated into respective device regions by STI (shallow trenchisolation) technology. That is, device isolation grooves 4 are made byRIE, and they are buried with a device isolation insulation film 5 asshown in FIG. 35. The polycrystalline silicon film 3 a will serve as abase layer of floating gates.

This method of first stacking the polycrystalline Si film as a part ofthe floating gates and thereafter making a groove-like device isolationregions into the Si substrate is a technique very effective forminiaturizing memory cells while alleviating variance of electricalproperties of the memory cells. The method of making floating gatesafter making the device isolation regions is liable to be affected byconcentration of an electric field near the device isolation regions,and also liable to invite variance in the amount of capacitance couplingbetween the floating gates and the Si substrate. To prevent theseproblems, the use of a process unsuitable for miniaturizing memory cellsis compelled.

Next stacked is a polycrystalline silicon film 3 b which will form anupper layer of the floating gates. Let the polycrystalline silicon film3 b be doped with phosphorus. As a result, in a later thermal step,phosphorus diffuses from the polycrystalline silicon film 3 b into theunderlying polycrystalline silicon film 3 a, and it results in uniformlydoping the impurity into the floating gates in form of a multi-layeredfilm. At that time, doping of a proper concentration of phosphorus willround corners of the floating concentration of the electric field toedges of the floating gates during write and erase operations.

Excessively high phosphorus concentration of the floating gates willadversely affect the tunnel oxide film 2 under the floating gates.Excessively low phosphorus concentration will leave lower corners of thefloating gates square, and will invite concentration of the electricfield. This will cause variance and deterioration in reliability ofwrite, erase and other properties of the memory cells. Therefore, propercontrol of the phosphorus concentration in the floating gates isimportant for flash memory. If arsenic is used as the impurity of thefloating gates, corner rounding by thermal oxidation is not expectedunlike the use of phosphorus, and phosphorus is used preferably.

After the step of FIG. 35(C), the polycrystalline silicon film 3 b isselectively etched to separate the film of the floating gates into cellregions, and thereafter, a gate insulation film 6 is formed and apolycrystalline silicon film 7 is stacked to form control gates.Commonly used as the gate insulating film 6 is a composite film (ONOfilm) of oxide/nitride/oxide layers.

Next directing to the peripheral circuit, in the status where the gateinsulating film 6 is formed in the cell array region, in the peripheralcircuit region, the gate insulating film is removed by etching, thepolycrystalline silicon films 3 a, 3 b are also removed, and the tunneloxide film is removed as well. Then, after an appropriate gate oxidefilm is formed to comply with a resistance to pressure necessary for therespective transistor regions, a polycrystalline silicon film 7 used ascontrol gates in the cell array region will be stacked. That is, bypatterning the polycrystalline silicon film 7, control gates in the cellarray region and gated electrodes of transistors in the peripheralcircuit are formed simultaneously.

After the control gates of the cell array and gate electrodes of theperipheral circuit are made, an n-type impurity is ion-implanted intothe cell array region and the NMOS transistor regions of the peripheralcircuit, and a p-type impurity is additionally ion-implanted into thePMOS transistor regions of the peripheral circuit. As a result, sourceand drain diffusion layers of the cell array region and the peripheralcircuit region are formed, the n-type impurity is doped into the controlgates of the cell array region and the gate electrodes of the NMOStransistor in the peripheral circuit, and the p-type impurity is dopedinto the gate electrodes of the PMOS transistor in the peripheralcircuit.

In the conventional process reviewed above, in the peripheral circuitregion needs removing the tunnel oxide film formed over both theperipheral circuit region-and the cell array region, newly forming agate oxide film for high-voltage circuit transistors, then selectivelyremoving the gate oxide film by etching and thereafter forming a gateoxide film for low-voltage circuit transistors. Repeating such etchingsteps of oxide films several times causes, in the peripheral circuitregion, retraction of the device isolation insulating film alreadyburied. FIG. 36(a) shows an aspect of such retraction. If the gate oxidefilm 8 is formed as shown in FIG. 36(b) on the structure shown in FIG.36(a) to make gate electrodes 9, edge portions of the gate electrodes 9enter into the concave portions of the device isolation insulating filmin contact with side surfaces of device regions as shown by the brokenline A.

Configuration as shown in FIG. 36(b) invites a short-channel effectopposite to a normal short-channel effect (opposite short-channeleffect), in which the threshold value lowers when the peripheral circuittransistors are short-channeled. Also invited are an increase of theleak current of the peripheral circuit transistors, deterioration oftheir sub-threshold characteristics and, hence, increase of the standbycurrent in the peripheral circuit. Further, deterioration of thereliability of the gate insulating film at end portions of the gateelectrodes is also invited.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a semiconductormemory integrated circuit and its manufacturing method that improveproperty and reliability of a peripheral circuit.

According to one aspect of the invention, there is provided asemiconductor memory integrated circuit comprising:

a semiconductor substrate;

a device isolation insulating film buried in grooves formed into thesemiconductor substrate;

a cell array having an arrangement of electrically erasable andprogrammable nonvolatile memory cells made by stacking floating gatesand control gates on the semiconductor substrate; and

a peripheral circuit disposed around the cell array on the semiconductorsubstrate,

at least the bottom layer of the floating gates of the nonvolatilememory cells and at least the bottom layer of gate electrodes oftransistors in the peripheral circuit being stacked before the deviceisolation insulating film is buried, then being maintained in selfalignment with the device isolation insulating film, and impuritiesbeing doped thereto under different conditions from each other.

According to another aspect of the invention, there is provided a methodof manufacturing a semiconductor memory integrated circuit comprisingthe steps of:

forming a plurality of gate insulating films each having a thicknessrequired for each of a cell array region and a peripheral circuit regionon a semiconductor substrate;

stacking a first-layer gate electrode material film not doped withimpurities on the gate insulating films;

etching the semiconductor substrate covered with the first-layer gateelectrode material film to make grooves for device isolation, andburying the device isolation grooves with a device isolation insulatingfilm;

stacking a second-layer gate electrode material film not doped withimpurities on the first-layer gate electrode material film maintained inself alignment with regions surrounded by the device isolationinsulating film and on the device isolation insulating film;

selectively introducing impurities into the first-layer and second-layergate electrode material films in the cell array region;

selectively etching the second-layer gate electrode material film toseparate the same on the device isolating insulating film in the cellarray region;

forming a gate insulating film on the second-layer gate electrodematerial film to serve as an insulation film between floating gates andcontrol gates of memory cells;

removing the gate insulating film from the peripheral circuit region;

stacking a third-layer gate electrode material film not doped withimpurities on the gate insulating film;

processing the gate electrode material in the memory cell region and theperipheral circuit region into a desired pattern to form control gateand floating gates in the memory cell region and form gate electrodes inthe peripheral circuit region; and

forming source and drain diffusion layers and lowering the resistance ofthe gate electrodes by introducing impurities into the memory cellregion and the peripheral circuit region under a plurality of differentconditions.

In the manufacturing method described above, as far as the cell arrayregion is concerned, floating gates can be also made solely of thefirst-layer gate electrode material film without stacking thesecond-layer gate electrode material film.

According to a further aspect of the invention, there is provided amethod of manufacturing a semiconductor memory integrated circuitcomprising the steps of:

forming a plurality of gate insulating film each having a thicknessrequired for each of a cell array region and a peripheral circuit regionon a semiconductor substrate;

stacking a first-layer gate electrode material film not doped withimpurities on the gate insulating films;

etching the semiconductor substrate covered with the first-layer gateelectrode material film to make grooves for device isolation, andburying the device isolation grooves with a device isolation insulatingfilm;

stacking a second-layer gate electrode material film not doped withimpurities on the first-layer gate electrode material film maintained inself alignment with regions surrounded by the device isolationinsulating film and on the device isolation insulating film;

selectively introducing impurities into the first-layer and second-layergate electrode material films in the cell array region;

selectively etching the second-layer gate electrode material film toseparate same on the device isolating insulating film in the cell arrayregion;

forming a gate insulating film on the second-layer gate electrodematerial film to serve as an insulation film between floating gates andcontrol gates of memory cells;

stacking a third-layer gate electrode material film on the gateinsulating film;

removing the third-layer gate electrode material film from theperipheral circuit region;

processing the gate electrode material in the memory cell region and theperipheral circuit region into a desired pattern to form control gatesand floating gates in the memory cell region and form gate electrodes inthe peripheral circuit region; and

forming source and drain diffusion layers and lowering the resistance ofthe gate electrodes by introducing impurities into the memory cellregion and the peripheral circuit region under a plurality of differentconditions.

According to a still further aspect of the invention, there is provideda method of manufacturing a semiconductor memory integrated circuitcomprising the steps of:

forming a plurality of gate insulating films each having a thicknessrequired for each of a cell array region and a peripheral circuit regionon a semiconductor substrate;

stacking a first-layer gate electrode material film not doped withimpurities on the gate insulating films;

selectively etching the semiconductor substrate covered with thefirst-layer gate electrode material film to form device isolationgrooves, and burying the device isolation grooves with a deviceisolation insulating film;

forming a barrier film on the device isolation insulating film and thefirst-layer gate electrode material film maintained in self-alignment inregions surrounded by the device isolation insulating film to preventdiffusion of impurities;

selectively removing the barrier film from above the cell array region;

stacking on the entire surface a second-layer gate electrode materialfilm doped with impurity;

selectively etching the second-layer gate electrode material film toremove the second-layer gate electrode material film from above thedevice isolation insulation film within the cell array region, andremoving the second-layer gate electrode material film from theperipheral circuit region;

stacking a third-layer gate electrode material film not doped withimpurities on the cell array region having the gate insulating filmselectively formed on the second-layer gate electrode material film andon the peripheral circuit region from which the barrier film has beenremoved;

selectively etching the first-layer and third-layer gate electrodematerial films to form control gates and floating gates in the cellarray region and gate electrodes in the peripheral circuit region; and

forming source and drain diffusion layers and lowering the resistance ofthe gate electrodes by introducing impurities into the memory cellregion and the peripheral circuit region under a plurality of differentconditions.

According to a yet further aspect of the invention, there is provided amethod of manufacturing a semiconductor memory integrated circuitcomprising the steps of:

forming a plurality of gate insulating films each having a thicknessrequired for each of a cell array region and a peripheral circuit regionon a semiconductor substrate;

stacking a first-layer gate electrode material film not doped withimpurities on the gate insulating films;

selectively etching the semiconductor substrate covered with thefirst-layer gate electrode material film to form device isolationgrooves, and burying the device isolation grooves with a deviceisolation insulating film;

doping an impurity into the first-layer gate electrode material film onthe cell array region;

etching the entire surface of the device isolation insulating filmprojecting upward to a level exposing side surfaces of the first-layergate electrode material film;

forming a gate insulating film to cover the first-layer gate electrodematerial film;

stacking a second-layer gate electrode material film over the entiresurface;

selectively etching the first-layer and second-layer gate electrodematerial films to form control gates and floating gates in the cellarray region and gate electrodes in the peripheral circuit region; and

forming source and drain diffusion layers and lowering the resistance ofthe gate electrodes by introducing impurities into the cell array regionand the peripheral circuit region under a plurality of differentconditions.

According to a yet further aspect of the invention, there is provided amethod of manufacturing a semiconductor memory integrated circuitcomprising the steps of:

forming a plurality of gate insulating films each having a thicknessrequired for each of a cell array region and a peripheral circuit regionon a semiconductor substrate;

stacking a first-layer gate electrode material film not doped withimpurities on the gate insulating films;

selectively etching the semiconductor substrate covered with thefirst-layer gate electrode material film to form device isolationgrooves, and burying the device isolation grooves with a deviceisolation insulating film;

forming a barrier film on the device isolation insulating film and thefirst-layer gate electrode material film maintained in self-alignment inregions surrounded by the device isolation insulating film to preventdiffusion of impurities;

selectively removing the barrier film from above the cell array region;

stacking on the entire surface a second-layer gate electrode materialfilm doped with impurity;

removing the entire surface of the second-layer, gate electrode materialfilm to the level of and exposing the top surface of the deviceisolation insulating film in the cell array region;

removing the second-layer gate electrode material film from theperipheral circuit region;

etching the entire surface of the device isolation insulating filmprojecting upward to a level exposing side surfaces of the first-layergate electrode material, film;

forming a gate insulating film to cover the second-layer gate electrodematerial film on the cell array region;

stacking a third-layer gate electrode material film on the entiresurface;

selectively etching the first-layer to third-layer gate electrodematerial films to form control gates and floating gates in the cellarray region and gate electrodes in the peripheral circuit region; and

forming source and drain diffusion layers and lowering the resistance ofthe gate electrodes by introducing impurities into the cell array regionand the peripheral circuit region under a plurality of differentconditions.

The present invention ensures impurity doping individually optimum forfloating gates and control gates of memory cells, and gate electrodes ofthe peripheral circuit. In addition, at least the bottom layer of gateelectrodes in the cell array region and the peripheral circuit region isstacked before the device isolation insulating film is buried, andremains in self-alignment with the device isolation insulating film.Therefore, unlike the process of making gate insulating films differentin thickness through a plurality of etching steps of oxide films afterburying the device isolation insulating film, here is preventedretraction of the device isolation insulating film in the peripheralcircuit region, and property and reliability of the peripheral circuittransistors can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is cross-sectional view that illustrates a step of making atunnel insulating film of flash memory according to the first embodimentof the invention;

FIG. 2 is a cross-sectional view that illustrates a step of making agate insulating film of a high-voltage circuit in the first embodiment;

FIG. 3 is a cross-sectional view that shows a step of selectivelyremoving a gate insulating film of a low-voltage circuit in the firstembodiment;

FIG. 4 is a cross-sectional view that illustrates a step of making thegate insulating film of the low-voltage circuit in the first embodiment;

FIG. 5 is a diagram illustrating that individual circuit regions in thefirst embodiment are covered with a first-layer polycrystalline siliconfilm;

FIG. 6 is a cross-sectional view that shows a step of burying a deviceisolation insulating film in the first embodiment;

FIG. 7 is a cross-sectional view that shows a step of stacking asecond-layer polycrystalline silicon film in the first embodiment;

FIG. 8 is a cross-sectional view that shows a step of ion-implanting animpurity into a cell array region in the first embodiment;

FIG. 9 is a cross-sectional view that shows a step of isolation offloating gates in the cell array region in the first embodiment;

FIG. 10 is a cross-sectional view that shows a step of making a gateinsulating film in the first embodiment;

FIG. 11 is a cross-sectional view that shows a step of removing the gateinsulating film in the peripheral circuit region in the firstembodiment;

FIG. 12 is a cross-sectional view that shows a step of stacking athird-layer polycrystalline silicon film in the first embodiment;

FIG. 13 is a cross-sectional view that shows a step of patterning gateelectrodes in the first embodiment;

FIG. 14 is a cross-sectional view that shows a salicide step in thefirst embodiment;

FIGS. 14A through 14D are cross-sectional views of a modification thatis partly modified from the first embodiment;

FIG. 15 provides cross-sectional views that show device structures ofdifferent circuit regions in the first embodiment;

FIG. 15A provides cross-sectional views that show device structures ofdifferent circuit regions in an example having a two-layered structure;

FIG. 16 is a cross-sectional view that shows a step of forming a blockfilm in the second embodiment;

FIG. 17 is a cross-sectional view that shows a step of selectivelyetching the block film in the second embodiment;

FIG. 18 is a cross-sectional view that shows a step of stacking asecond-layer polycrystalline silicon film in the second embodiment;

FIG. 19 is a cross-sectional view that shows a step of selective etchingthe second-layer polycrystalline film in the second embodiment;

FIG. 20 is a cross-sectional view that shows a step of making a gateinsulating film in the second embodiment;

FIG. 21 is a cross-sectional view that shows a step of etching the gateinsulation film and the block film in the second embodiment;

FIG. 22 is a cross-sectional view that shows a step of stacking athird-layer polycrystalline silicon film in the second embodiment;

FIG. 23 is a cross-sectional view corresponding to FIG. 6, which shows astep in the third embodiment of the invention;

FIG. 24 is a cross-sectional view that shows a step of ion-implantationto the first-layer polycrystalline silicon film in the cell array regionin the third embodiment;

FIG. 25 is a cross-sectional view that shows a step of etching an oxidefilm in the third embodiment;

FIG. 26 is a cross-sectional view that shows a step of making a gateinsulating film in the third embodiment;

FIG. 27 is a cross-sectional view that shows a step of etching the gateinsulating film in the peripheral circuit region in the third embodimentof the invention;

FIG. 28 is a cross-sectional view that shows a step of stacking asecond-layer polycrystalline silicon film in the third 5 embodiment;

FIG. 29 is a cross-sectional view corresponding to FIG. 18, which showsa step in the fourth embodiment of the invention;

FIG. 30 is a cross-sectional view that shows a step of smoothing asecond-layer polycrystalline silicon film in the fourth embodiment;

FIG. 31 is cross-sectional view that shows a step of etching thesecond-layer polycrystalline silicon film in the peripheral circuitregion in the fourth embodiment;

FIG. 32 is a cross-sectional view that shows a step of making a gateinsulating film in the fourth embodiment;

FIG. 33 is a cross-sectional view that shows a step of etching the gateinsulating film in the peripheral circuit region in the fourthembodiment;

FIG. 34 is a cross-sectional view that shows a step of stacking athird-layer polycrystalline silicon film in the fourth embodiment;

FIG. 35 is a cross-sectional view that shows a manufacturing process ofa cell array section in conventional flash memory; and

FIG. 36 is a diagram for explaining a problem with peripheral circuittransistors in the conventional flash memory.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be explained below with reference tothe drawings.

First Embodiment

FIGS. 1 through 13 show a manufacturing process of flash memoryaccording to an embodiment of the invention. In this embodiment, asshown in FIG. 1, n-type wells 11 and p-type wells 12 necessary forrespective circuit regions of a silicon substrate 10 are formed beforeisolation into devices. In addition, in this embodiment, a plurality ofgate insulating films having different values of thickness necessary forrespective circuit regions are made before STI (Shallow Trenchisolation) formation into devices.

For example, as shown in FIG. 1, a tunnel oxide film 21 a is first madeas a gate insulating film of a thickness around 8 nm necessary for thecell array region. Since tunnel oxide films, in general, may neednitrification, they are first formed and undergo necessary processing.After that, as shown in FIG. 2, a polycrystalline silicon film 22 a isstacked, and then selectively removed by etching to remain only in thecell array region. The polycrystalline silicon film 22 a is not dopedwith impurities, and will form the bottom layer of floating gates in thecell array region. While protecting the cell array region with thepolycrystalline silicon film 22 a, thermal oxidation is conducted toform a gate oxide film 21 b necessary for a high-voltage circuit in theperipheral circuit region. Thereafter, ion implantation is conducted inthe peripheral circuit region for controlling the channel impurityconcentration.

Subsequently, as shown in FIG. 3, the cell array region and thehigh-voltage circuit region in the peripheral circuit region are coveredwith a resist 23, for example, and the gate insulating film 21 b isselectively removed by etching from a low-voltage circuit region in theperipheral circuit region. Thereafter, thermal oxidation is conducted toform a gate oxide film 21 c required in the low-voltage circuit regionas shown in FIG. 4. For example, if the gate oxide film 21 b of thehigh-voltage circuit requires the thickness of 17 nm, it is initiallymade to be approximately 14 nm thick. If the gate oxide film 21 c of thelow-voltage circuit is formed to a thickness around 8 nm, during thisoxidation process, thickness of the gate oxide film 21 b of thehigh-voltage circuit is increased to about 17 nm.

After that, a polycrystalline silicon film 22 b is stacked as shown inFIG. 5. The polycrystalline silicon film 22 b will become the bottomlayer of gate electrodes in the peripheral circuit region, and let it benot doped with impurities in this stage. This polycrystalline siliconfilm 22 b is stacked also on the polycrystalline silicon film 22 a inthe cell array region; however, this is removed from there. Thusobtained is the structure as shown in FIG. 5, having respectivelynecessary gate oxide films 21 a, 21 b and 21 c formed in the respectivecircuit regions, and covered with the non-doped polycrystalline siliconfilms 22 a, 22 b. Heretofore, although two-layered polycrystallinesilicon films 22 a, 22 b are used, they will form the bottom layers offloating gates in the cell array region and gate electrodes in theperipheral circuit region, respectively, and they are hereinbelowcollectively referred to as a first-layer polycrystalline silicon film.

An object of the invention lies in entrainment of a high-performanceperipheral circuit in flash memory, a difference will be produced incontrollability of the impurity profile, depending upon which of stepsion implantation is conducted in for controlling channel impurityprofiles of transistors in the peripheral circuit. The method ofinjecting it at the stage explained in the foregoing embodiment is anexample. In another example, ions may be implanted through thepolycrystalline silicon film 22 after obtaining the configuration ofFIG. 5. Any of these examples, impurity diffusion in channel regions inthe peripheral circuit can be decreased in thermal and oxidation stepsas compared with a method in which all ion implantation is previouslyconducted to the tunnel oxide film of FIG. 1, and performance ofperipheral circuit transistors is enhanced.

The foregoing steps up to that of FIG. 5 can be modified by, forexample, adding some steps like a step of protecting the surface of thepolycrystalline silicon film 22 a with a nitride film.

After that, as show in FIG. 6, device isolation grooves 13 are made byRIE, and buried with device isolation insulating film 14. In this deviceisolation step, for example, a mask in form of a multi-layered film ofsilicon nitride film and a silicon oxide film (not shown) stacked on thefirst-layer polycrystalline silicon film 22 is used, and it is removedafter the structure is smoothed by burying the device isolationinsulating film 14. As a result, as shown in FIG. 6, the first-layerpolycrystalline silicon film 22 is divided into respective memory cellregions and transistor regions in a self-aligned manner with the deviceisolation regions.

After that, as shown in FIG. 7, a second-layer polycrystalline siliconfilm 24 is stacked. This second-layer polycrystalline silicon film 24 isalso non-doped. Then, as show in FIG. 8, a resist 25 is having anaperture in the cell array region is formed by patterning, thenphosphorus is ion-implanted into the second-layer polycrystallinesilicon film 24 in the cell array region and later diffused to thefirst-layer polycrystalline silicon film 22. At that time,ion-implanting conditions are adjusted such that impurity concentrationsof the first-layer polycrystalline silicon film 22 and the second-layerpolycrystalline silicon film 24 become relatively high concentrations inthe first half of 1020/cm3.

However, phosphorus introduced by ion implantation to a highconcentration is liable to cause channeling, and might damage the tunneloxide film or penetrate into the underlying substrate, thereby to affectthe control of the threshold value. In addition, ion implantation mightresult in driving metal or other impurity into the floating gates, whichwill invite abnormal leakage through the tunneling oxide film ordegradation of reliability of a gate insulating film that will be formedlater around the floating gates. Therefore, particular attention has tobe paid to acceleration voltage and other conditions for ionimplantation. A method of overcoming these problems will be explainedlater with another embodiment.

After that, through a lithographic step, the second-layerpolycrystalline silicon film 24 in the cell array region is selectivelyetched to separate it on the device isolation regions as shown in FIG.9. In the cell array region, the multi-layered film of the first-layerpolycrystalline silicon film 22 and the second-layer polycrystallinesilicon film 24 will form floating gates. At this stage, however, in thedirection normal to the drawing sheet, isolation of floating gates intoindividual memory cells is not conducted.

After that, as shown in FIG. 10, a gate insulating film 26 is formed onthe entire surface of the substrate for the purpose of separatingfloating gates and control gates to be formed thereon into discretememory cells. The gate insulating film 26 is an ONO film. Then, as shownin FIG. 11, a resist 27 covering the cell array region is formed bypatterning to remove the gate insulating film 26 from the peripheralcircuit region by etching.

After that, as shown in FIG. 12, a third-layer polycrystalline siliconfilm 28 is stacked on the entire surface. This third-layerpolycrystalline silicon film 28 is also a non-doped film, which willform the control gates in the cell array region and the top layers ofthe gate electrodes in the peripheral region. In the peripheral circuitregion, three layers of the non-doped polycrystalline silicon film 22,24, 28 result in having been stacked in contact with each other.

Subsequently, gate electrode processing in each region is conducted. Asshown in FIG. 13, in the cell array region, the third-polycrystallinesilicon film 28 is patterned to form lines of control gates that arecontinuous as word lines, and in self alignment with them, thesecond-layer polycrystalline silicon film 24 and the first-layerpolycrystalline silicon film 22 are patterned and separated intofloating gates of individual memory cells in the direction normal to thedrawing sheet. In the peripheral circuit region, the third-layerpolycrystalline silicon films 22, 24, 28 are patterned into gateelectrodes and gate wirings. FIG. 13 shows that gate electrodes G11 ofPMOS transistors and gate electrodes G12 of NMOS transistors have beenformed by patterning in the high-voltage circuit whereas gate electrodesG21 of PMOS transistors and gate electrodes G22 of NMOS transistors havebeen formed by patterning in the low-voltage circuit.

After that, ions are introduced into the respective circuit regionsunder different conditions to obtain desired conduction types andoptimum impurity concentrations for respective gate electrodes to forsource and drain diffusion layers. Conditions therefor will be describedlater. After that, as shown in FIG. 14, a salicide film 29 of a metalhaving a high melting point, such as Co, for the purpose of lowering theresistance, is formed on surfaces of the diffusion layers and the gateelectrodes.

A modification of the foregoing embodiment will be explained below. Inthe foregoing embodiment, as shown in FIG. 11, after the gate insulatingfilm 26 is stacked and removed from the peripheral circuit region, thethird layer-polycrystalline Si film 28 is stacked to serve as controlgates in the cell array region. Further, in the peripheral circuit, thefirst-layer polycrystalline Si film 22, second-layer polycrystalline 24and third-layer polycrystalline Si film 28 are in direct contact tofunction as gate electrodes in the peripheral circuit region. It isfurther decrease here that, even when a photoresist is coated on thegate insulating film 26, reliability of the insulating film is notdegraded. It is also desirable to decrease the height of the gateelectrodes of the peripheral transistors. For these purposes, here isproposed the modification shown below.

This modification is the same as the foregoing embodiment up to the stepof FIG. 10. After the step of FIG. 10, however, the polycrystallinesilicon film 28 is stacked on the entire surface as shown in FIG. 14A.Next as shown in FIG. 14B, the third-layer polycrystalline silicon film28 and the underlying gate insulating film 26 are removed by etchingfrom the peripheral region, which are central and right areas in thedrawing. After that, as shown in FIG. 14C, two layers of thepolycrystalline silicon films 22, 24 are patterned into the form ofrespective gate electrodes and gate wirings. G11, G12, G21 and G22 inFIG. 14C correspond to those in FIG. 13. Subsequent processing of ionimplantation to obtain desired conduction types and optimum impurityconcentrations of respective gate electrodes and making source and draindiffusion layers is the same as shown in FIG. 14. Then, as shown in FIG.14D, the salicide film 29 of a metal having a high melting point, suchas Co, for the purpose of lowering the resistance, is formed on surfacesof the diffusion layers and the gate electrodes.

With the structure explained above, since the top surface of the gateinsulating film is protected by the third-polycrystalline Si film, itsreliability does not deteriorate. Further, in the peripheral circuitregion, the polycrystalline Si film of the gate electrodes results incomprising the continuous two layers, i.e. the first-layerpolycrystalline Si film 22 and the second-layer polycrystalline Si film24. Therefore, height of the gates is lower, and they are more easilyprocessed than those of the foregoing embodiment. Transistors with sucha two-layered structure are shown in FIG. 15A, which correspond to FIG.15.

An example of cross-sectional structures of transistors of a memory celland the peripheral circuit region are explained below with reference toFIG. 15. The structure is characterized in the configuration of the gateelectrodes.

In both the peripheral circuit region and the cell array region, thereare provided source and drain extension regions with relatively lowimpurity concentrations, and source and drain regions with high impurityconcentrations (LDD structure). Before making side walls 36, impuritiesare introduced into the extension regions (N−, P−), and after making theside walls 36, impurities are introduced into the regions with highimpurity concentrations (N+, P+). Normally, the gate electrodes aresimultaneously doped upon ion implantation into regions with a highimpurity concentration. Amount of this ion implantation may be around3×1015 cm-2. Amount of implantation in the extension regions istypically not higher than 1×1015 cm-2. In general, materials ofimpurities and their concentrations are different among N-typeextensions and high-concentrated diffusion layers in memory cells andthe peripheral circuit region.

In general, as shown in FIG. 15, individual gate electrodes are coveredwith an oxide film 35 by post-oxidation to make a side wall insulatingfilm 36 of a silicon nitride. Then, surfaces of the gate electrodes anddiffusion layers are exposed, a metal with a high melting point isstacked, and thereafter, the salicide layer 29 is formed on the gateelectrodes and the diffusion layers by annealing to obtain a salicidestructure.

Memory cells and peripheral transistors are not limited to thosestructures; depending upon the types of and required performance of thememory cells. Memory cells, for example, may have only N-regions in NANDflash memory as shown in FIG. 15. In this case, the process may bemodified to prevent that a high-concentrated diffusion layer or silicidelayer is formed in portions of the N-layer.

The third gate electrode, stacked, may be doped with an N-type impurity.

Through those steps, the process of making devices is completed.Subsequently, although not shown, an inter-layer insulating film isstacked, contact holes are made, and metal wirings including bit linesand source lines are made.

According to the embodiment explained above, in which device isolationby STI technique is executed after the tunnel insulating film in thecell array region and the gate insulating film of high-voltagetransistors and low-voltage transistors in the peripheral circuit aremade under their optimum conditions, it is possible to preventretraction of the buried insulating film in the peripheral circuitregion and deterioration of the transistor characteristics in theperipheral circuit, which are problems involved in the conventionaltechnique configured to repeat etching of oxide films after isolation ofdevices.

Further, since the floating gates in the cell array region have atwo-layered polycrystalline silicon structure whereas the gateelectrodes of peripheral circuit transistors have a three-layeredpolycrystalline silicon structure, both formed under individuallyoptimum conditions for introducing impurities (conduction types andconcentrations), more stable write and erase characteristics in the cellarray, higher performance of the peripheral circuit, and higherreliability of the, flash memory are ensured.

Especially in case of the instant embodiment, in which phosphorus isused as the impurity of the floating gates in the cell array, sincecorners of the floating gates are rounded thereby in a later oxidationstep, concentration of electric fields is prevented during write anderase operations using highly raised potentials. Therefore, fluctuationamong individual cells is prevented, and excellent write and erasecharacteristics required for chips, such as enabling a tight write anderase profile, can be obtained. As to NMOS transistors in the peripheralcircuit, arsenic is doped into the source, drain diffusion layers andthe gate electrodes, and high-performance transistors with shallowdiffusion layers can be obtained.

Second Embodiment

A manufacturing process of another embodiment of the invention will beexplained below with reference to FIGS. 16 through 22. The steps shownin FIGS. 1 through 6 are common to the instant embodiment. That is, alsoin this embodiment, optimum gate insulating films are formedrespectively in the cell array region, high-voltage circuit region andlow-voltage circuit region of the peripheral circuit, the non-dopedfirst-layer polycrystalline silicon film 22 is formed thereon, and thedevice isolation insulating film 14 is buried.

After the step of FIG. 6, the instant embodiment takes the step of FIG.16 in which a barrier film (block film) 41 is formed on the entiresurface to function as both a barrier against impurity diffusion and anetching stopper used as this barrier film 41 is, for example, a siliconoxide film by CVD, but a silicon nitride film is also usable. Afterthat, as shown in FIG. 17, a resist 42 having an aperture in the cellarray region is formed by patterning, and the barrier film 41 isselectively removed from the cell array region by etching using theresist 42 as a mask.

After that, as shown in FIG. 18, the second-layer polycrystallinesilicon film 24 is stacked on the entire surface. Let this second-layerpolycrystalline silicon film 24 be polycrystalline silicon doped withphosphorus during deposition unlike that of the first embodiment. Thusthe second-layer polycrystalline silicon film 24 is in direct contactwith the first-layer polycrystalline silicon film 22 merely in the cellarray region. In a later thermal step, phosphorus diffused from thesecond-layer polycrystalline silicon film 24 into the first-layerpolycrystalline silicon film 22, and the resulting composite film willform floating gates in the cell array region. In the peripheral circuitregion, diffusion of phosphorus from the second-layer polycrystallinesilicon film 24 into the first-layer polycrystalline silicon film 22 isblocked by the barrier film 41.

After that, as shown in FIG. 19, a resist pattern 43 having openings inthe device isolation regions in the cell array region and having anopening all over the peripheral circuit region is formed by patterning,and the second-layer polycrystalline silicon film 24 is selectivelyetched. As a result, the first-layer polycrystalline silicon film 22 andfloating gates made up of the second-layer polycrystalline silicon layer24 are separated on the device isolation regions in the cell arrayregion whereas the second-layer polycrystalline silicon film 24 isremoved in the peripheral circuit region.

After that, as shown in FIG. 20, the gate insulating film 26 is formedfor separating floating gates and control gates in the cell arrayregion. Like the foregoing embodiment, let the gate insulating film 26be an ONO film. Then as shown in FIG. 21, a resist 44 is formed in apattern covering the cell array region, and the gate insulating film 26is removed by etching together with the underlying barrier film 41 fromthe peripheral circuit region by etching.

Subsequently, as shown in FIG. 22, the third-layer polycrystallinesilicon film 28 is stacked the third-layer polycrystalline silicon film28 is a non-doped film similarly to the foregoing embodiment.Subsequently, through the same steps as those of FIG. 13, et seq.already explained with reference to the first embodiment, gateelectrodes are formed by patterning in respective circuit regions, ionimplantation is conducted under different conditions for respectivecircuit regions, optimum conduction types and optimum impurityconcentrations are adjusted for gate electrodes, and source and draindiffusion layers are formed. Conditions for these steps are the same asthose of the first embodiment. After that, a salicide film of a metalhaving a high melting point, such as Co, is preferably formed onsurfaces of the diffusion layers and gate electrodes for the purpose oflowering the resistance.

The instant embodiment is also configured to execute device isolation bySTI technique after making the tunnel insulation film in the cell arrayregion and gate insulation film of high-voltage transistors andlow-voltage transistors in the peripheral circuit under theirrespectively optimum conditions, and thereby again preventsdeterioration of the transistor characteristics due to retraction of theburied insulating film in the peripheral circuit region.

In addition, the instant embodiment does not use ion implantation uponintroducing phosphorus into the floating gates of memory cells. That is,phosphorus is doped into the second-layer polycrystalline silicon filmwhen stacking it, and it is diffused by solid-phase diffusion into thefirst-layer polycrystalline silicon film forming the bottom layer of thefloating gates. Therefore, damage to the tunnel oxide film and otheradverse affects by channeling are prevented unlike the process relyingon high-concentrated phosphorus ion implantation.

Furthermore, although both the floating gates in the cell array regionand the gate electrodes of the peripheral circuit transistors have atwo-layered polycrystalline silicon structure, since they are made underrespectively optimum conditions for introducing impurities (conductiontypes and concentrations), stable write and erase properties of cellarrays, high performance of the peripheral circuit and high reliabilityof the flash memory are ensured.

Third Embodiment

A further embodiment modified from the process of the first embodimentto separate floating gates in a self-aligned manner is explained withreference to FIGS. 23 through 28. The steps shown in FIGS. 1 through 6are common to the instant embodiment. Although FIG. 23 corresponds toFIG. 6, the side configuration of the device isolation insulating film14 in the instant embodiment preferably extends fully vertical, and FIG.23 shows it as having vertically extending side surfaces.

After that, as shown in FIG. 24, a resist 51 having an aperture in thecell array region is formed by lithography, and phosphorus ision-implanted into the first-layer polycrystalline silicon film 22,which will form floating gates in the cell array region. The deviceisolation insulating film 14 is next etched entirely to a level exposingside surfaces of the first-layer polycrystalline silicon film 22 asshown in FIG. 25. Subsequently, as shown in FIG. 26, the gate insulatingfilm 26 in form of ONO film is formed.

Next as shown in FIG. 27, a resist 52 is formed in a pattern having anaperture in the peripheral circuit region by lithography, and the gateinsulating film 26 is removed by etching from the peripheral circuitregion. Next as shown in FIG. 28, the polycrystalline silicon film 24 isstacked as a second-layer gate electrode material film on the entiresurface. The polycrystalline silicon film 24 will form control gates inthe cell array region, and will constitute, together with thefirst-layer polycrystalline silicon film 22, gate electrodes in theperipheral circuit region. Subsequent steps follow those of the firstembodiment.

According to the instant embodiment, it is possible to not onlyintegrate high-performance transistors in the peripheral circuit butalso reduce the cell size in the cell array region because of isolationof floating gates comprising solely of the first-layer polycrystallinefilm 22 in a self-aligned manner. Additionally, although the floatinggates made up of the first-layer polycrystalline silicon film 22 doesnot extend into the device isolation region, since the control gates areopposed also to their side surfaces, coupling capacitance between thecontrol gates and the floating gates can be enhanced.

Fourth Embodiment

A further embodiment modified from the second embodiment to separatefloating gates in a self-aligned manner is explained with reference toFIGS. 29 through 34. Steps of the second embodiment up to the step ofFIG. 18 are common to the instant embodiment. Although FIG. 29corresponds to FIG. 18, the side configuration of the device isolationinsulating film 14 in the instant embodiment preferably extends fullyvertical, and FIG. 29 shows it as having vertically extending sidesurfaces.

After that, upper part of the second-layer polycrystalline silicon film24 containing phosphorus is removed by CMP. As a result, as shown inFIG. 30, in the cell array region, the second-layer polycrystallinesilicon film 24 is maintained in a self-aligned manner solely in thememory cell regions interposed between separate adjacent regions of thedevice isolation insulating film 14 and used together with thefirst-layer polycrystalline silicon film 22 to form floating gates.After that, as shown in FIG. 31, while covering the cell array regionwith a resist 61, the second-layer polycrystalline silicon film 24 stillremaining the peripheral circuit region is removed by CDE (Chemical DryEtching).

Subsequently, the entire substrate surface undergoes etching of oxidefilms including the barrier film 41 and the device isolation insulatingfilm 14 such that the top surface of the device isolation insulatingfilm retracts in the cell array region at least until side surfaces ofthe second-layer polycrystalline silicon film 24 are exposed. Then asshown in FIG. 32, the gate insulating film 26, which is a ONO film, isformed for the purpose of separating floating gates and control gates ofmemory cells. Next as shown in FIG. 33, a resist 62 is formed to coverthe cell array region by lithography, and the gate insulating film 26 isremoved by etching from the peripheral region: Subsequently, as shown inFIG. 34, the third-layer polycrystalline silicon film 28 is stacked onthe entire surface to form the control gates of the memory cells andpartly form the gate electrodes of transistors of the peripheralcircuit. Subsequent steps follow those of the second embodiment.

The instant embodiment also enables integration of high-performancetransistors in the peripheral circuit and contributes to reduction ofthe cell size in the cell array region because of isolation of floatinggates made of the first-layer polycrystalline silicon film 22 and thesecond-layer polycrystalline silicon film 24 in a self-aligned manner.Additionally, although the floating gates made of the first layerpolycrystalline silicon film 22 and the second-layer polycrystallinesilicon film 24 do not extend into the device isolation regions, sincethe control gates are opposed also to their side surfaces, couplingcapacitance between the control gates and the floating gates can beenhanced.

The invention is not limited to the embodiments explained above. Forexample, although the foregoing embodiments have been explained aspolycrystalline silicon films as gate electrode material films,amorphous silicon films may be used as an alternative.

As described above, according to the invention peripheral circuittransistors can be prevented from deterioration of properties due toretraction of the device isolation insulating film by first making thegate insulating film required in respective circuit regions before thestep of isolating devices by STI such that the gate insulating film iscovered by the bottom material layer of the gate electrodes. Moreover,by doping the floating gates in the cell array and the gate electrodesof transistors in the peripheral circuit region with impurities underrespectively optimum conditions, high-performance flash memory can beobtained.

In the specification, polycrystalline Si has been referred to often asbeing non-doped, or being not doped, with impurities; however, it willbe appreciated from the concept of the invention that suchpolycrystalline Si may be doped with impurities by a sufficient lowconcentration that the final doping concentration of impurities to thegate electrodes.

1-8. (canceled)
 9. A method of manufacturing a semiconductor memoryintegrated circuit comprising: forming a plurality of gate insulatingfilms each having a thickness required for each of a cell array regionand a peripheral circuit region on a semiconductor substrate; forming afirst-layer gate electrode material film not doped with impurities onsaid gate insulating films; etching said semiconductor substrate coveredwith said first-layer gate electrode material film to make grooves fordevice isolation, and burying the device isolation grooves with a deviceisolation insulating film; forming a second-layer gate electrodematerial film not doped with impurities on said first-layer gateelectrode material film maintained in self alignment with regionssurrounded by said device isolation insulating film and on said deviceisolation insulating film; selectively introducing impurities into saidfirst-layer and second-layer gate electrode material films in said cellarray region; selectively etching said second-layer gate electrodematerial film to isolate same on said device isolation insulating filmin said cell array region; forming a gate insulating film on saidsecond-layer gate electrode film to serve as an insulation film betweenfloating gates and control gates of memory cells; removing said gateinsulating film from said peripheral circuit region; forming athird-layer gate electrode material film not doped with impurities onsaid gate insulating film; processing the gate electrode material insaid memory cell region and said peripheral circuit region into adesired pattern to form control gate and floating gates in said memorycell region and form gate electrodes in said peripheral circuit region;and forming source and drain diffusion layers and lowering theresistance of said gate electrodes by introducing impurities into saidmemory cell region and said peripheral circuit region under a pluralityof different conditions.
 10. The method of manufacturing a semiconductormemory integrated circuit according to claim 9 wherein the step ofselectively introducing impurities into said first-layer andsecond-layer gate electrode material films is ion-implantation ofphosphorous.
 11. The method of manufacturing a semiconductor memoryintegrated circuit according to claim 9 wherein the step of formingsource and drain diffusion layers and lowering the resistance of saidgate electrodes includes ion implantation of arsenic into said cellarray region and NMOS transistor regions in said peripheral circuitregion, and ion implantation of boron into PMOS transistor regions insaid peripheral circuit region. 12-20. (canceled)